There has always been a need for high volume screening assays to identify the presence or absence of antigenic materials, antibodies and analytes in a large number of different sampling situations. Various test methods have been used in the past including gas chromatography, mass spectrometry, liquid chromatography and various bioassay methods. Often these methods are time consuming, expensive and cannot be applied to large scale screening programs in an efficient manner.
It has been suggested that immunoassay methods could be used for such screening since immunoassays are known to be easily designed to be specific, highly sensitive and simple to perform. Radioimmunoassays for example have found a large market and use in connection with clinical diagnostics. However, RIA procedures are often incompatible with large scale screening programs. Radiotracers used have inherently limited stability and special disposal and personnel screening procedures are often required. Sophisticated instrumentation is often necessary. For certain uses RIA may create potential hazards as in food processing environments.
Other techniques have been developed such as fluorescent or enzymatic immunoassay techniques which are useful in that potentially hazardous reagents are avoided. However, often these methods require separation by filtration or centrifugation steps in procedures used. Such separations make test procedures inherently slower and difficult to automate.
In a more recent development, enzyme labeled antigen is used which requires no bound-free separation and thus can be performed quickly with excellent sensitivity. Such a system can be automated for high volume assays as in EMIT system disclosed by Rosenthal, A. F., Vargas, M. G. and Klass, C. S. (1976) Clin Chem. 22, 1899. This system utilizes a mode of coupling antigen to enzyme which is quite critical and can result in the system being not readily adapted to different analyses without extensive development for each new system.
Recently, there have been reports of liposomes which can carry enzymes or substrates and be labeled with antigens or antibodies. Liposomes labeled with antigens at their external surface and containing an enzyme entrapped in their internal volume are reportedly mixed with cognate antibody and complement to determine whether or not the liposomes permit release of the entrapped enzyme. This determination is reported made by detecting enzymatic activity which is physically released from the liposomes after separating liposomes from surrounding medium. See Uemura, K. and Kinsky, S. C. (1972) Biochemistry, 11, 4085-4094 and Kataoka, T., Williamson, J. and Kinsky, S. (1973) Biochemics et Biophysica Acta 298, 158-179. However, there has been no recognition that such liposomes when suitably formed with suitable high signal to noise ratios can be useful for immunoassay procedures which avoid the use of separation steps and permit testing in homogeneous phase reactions. Moreover there are reported difficulties in preparing prior art immunospecific liposomes, G. H. Strejan, P. M. Smith, C. W. Grant and D. Surlan, "Naturally Occurring Antibodies to Liposomes", The Journal of Immunology, Vol. 123, No. 1, July 1979, 370-378. Furthermore, it has long been established that diffusion of macromolecules such as enzymes through lesions produced by complement in bilayer membranes is very much slower than that of small molecules (Green, H., Barrow, P. and Goldberg, B. [1959] J. Exp. Med. 110, 699).